Ponding On Flat Roofs And Drainage Loads

Water on a roof is heavy. A hundred millimetres of standing water over a roof area weighs about the same as a hundred kilograms per square metre, which is a substantial fraction of what many light roofs were designed to carry.

The dangerous part is that it is self reinforcing. Water collects in a low point, the extra weight deflects the roof, the deflection deepens the low point, and more water collects.

Why flat roofs are not flat

A roof described as flat has a fall, and the fall is what makes it work.

Which means the drainage design and the structural design are the same design. Three things decide whether water leaves.

The fall, set so water reaches an outlet rather than sitting in the middle of a bay.

The outlets, sized and positioned for the catchment and the rainfall intensity.

The overflow provision, which is the part most often missing and the part that matters most.

A roof with adequate falls and adequate outlets still ponds if the outlets block, and outlets block routinely from leaves, debris, construction material and bird nests.

Overflows are the structural control

The overflow is what limits how deep the water can get when the primary drainage fails.

Which makes it a structural safeguard rather than a plumbing nicety: it sets the maximum head of water the roof can ever experience, and therefore the maximum load.

A roof with a parapet, blocked outlets and no overflow has no limit at all other than the height of the parapet. Which is how roofs collapse in a storm, and it is almost always a maintenance failure sitting on top of a detailing omission.

So the detail is a scupper or an overflow outlet through the parapet, set at a level that is comfortably within the roof's capacity, sized to discharge the full design flow on its own, and positioned so the discharge is visible. Water pouring out of a parapet during a storm is how a blocked primary outlet gets reported.

Deflection has to be accounted for

The fall on the drawing is the fall on a perfectly flat structure. The fall in reality is the drawing fall minus the deflection of the roof under its own load, plus the long term creep in a concrete roof.

Which means a shallow fall on a long span can be entirely cancelled out by deflection, producing a dead flat or reverse fall at mid span. This is why minimum falls exist and why they are larger than they look necessary.

On a concrete roof, creep continues for years, so a roof that drained on completion can pond in year five. The mechanism is in [shrinkage and creep in concrete structures](/blog/shrinkage-and-creep-in-concrete-structures), and the serviceability limits are in [deflection limits and serviceability](/blog/deflection-limits-and-serviceability).

Precamber is the usual answer on a long span steel roof, and the fall is set on top of the cambered profile rather than on the flat.

Where ponding turns up

Box gutters, which are the highest risk element on any roof. They carry a large catchment in a confined channel, and a blocked box gutter with inadequate overflow is a concentrated load along a line plus a guaranteed leak into the building.

Behind a parapet, where the water has nowhere to go sideways.

Roof terraces and green roofs, where the drainage is under a surface nobody can see and the saturated weight is the design case. That is the loading problem in [green roofs and roof terrace loading](/blog/green-roofs-and-roof-terrace-loading).

Around new rooftop plant, where a plinth or a frame has interrupted the fall and created a dam. A common consequence of adding equipment later, alongside the capacity question in [rooftop solar and plant structural adequacy](/blog/rooftop-solar-and-plant-structural-adequacy).

At the low end of a long shallow span, where deflection has eaten the fall.

On a light roof the margin is small

A steel framed light roof has very little dead load, which makes it efficient and makes added water a large proportional increase.

Which is why the ponding check matters more on a warehouse or an awning than on a thick concrete roof, and why a light roof with a parapet and no overflow is a specific thing to look for on any existing building assessment.

Investigating an existing roof

The questions are straightforward and they are rarely asked.

Where does the water actually go, confirmed by looking at the roof during rain rather than reading the drawings. Are there overflows, and at what level. Is there standing water now, and how deep. Has the roof deflected since it was built, measured with a level rather than judged by eye. And what has been added to it since.

Standing water visible on a dry day is the clearest signal there is, because it means the roof has been holding that load long enough to be noticed, and the structure beneath it has been carrying it the whole time.

Where that is found, the remedy is usually drainage rather than structure: new outlets, new overflows, and a regular clearing regime. Where the deflection is already significant, it becomes a structural assessment as well.

ACSES provides structural engineering and roof assessment across Sydney. Talk to us about a project.